This proposal addresses two fundamental questions concerning normal development: (1) how a cell arrives at a specific state of determination as a function of its position within the embryo, and (2) how this specific state of determination is passed by the cell to its descendents. Electron microscopic studies of chromatin have suggested that DNA replication temporarily perturbs transcriptional termination signals in nonribosomal (but not in ribosomal) DNA sequences. This perturbation appears to allow active RNA polymerase molecules to read beyond the usual termination signal of a transcription unit and on into adjoining DNA sequences, transiently, during S-phase. This replication-dependent transcriptional readthrough could activate new control genes and hence provide a stochastic mechanism for the stepwise passage of dividing cells through a succession of determined states. This possibility will be explored by an analysis of electron micrographs of spread chromatin from precisely staged individual Drosophila embryos. In Drosophila the first 13 cycles of division of the embryonic nuclei occur with near synchrony, but this synchrony is lost during the 14th cycle, when the cells become determined. Then clusters of 5-30 cells enter mitosis together; these clusters are distributed over the embryonic surface in a specific and bilaterally symmetrical pattern. Each cluster may be made up of cells that share a common state of determination. This hypothesis will be tested by time-lapse video studies of normal embryos and of three mutants (bicaudal, dorsal-D and Toll), that are known to produce global changes in cell fate. Studies of fixed and stained whole embryos will supplement this investigation.